Automatic gauge control system for rolling mills



Aug. 16, 1966 w. ca. WRIGHT 3,256,279

AUTOMATIC GAUGE CONTROL SYSTEM FOR ROLLING MILLS Filed Oct. 30. 1963 4 Sheets-Sheet 1 (0 U2 hl T Direction strip 2 5 I travels of Vh. sec. g I

I 6 L". I

FIG. I

INVENTOR. WILLIAM QWRIGHT ATTORNEY Aug. 16,1966

Filed Oct. 30, 1963 W. G. WRIGHT AUTOMATIC GAUGE CONTROL SYSTEM FOR ROLLING MILLS (a) LOAD J24 CELL SIGNAL ,2

OUTPUT 2 (b) FIG.3

T RAuszer r .DEL 9 I I 3 h) 5 r'--- M l I l m J 3 0 (by MEASURED (c) GAUGE ROLL 3| (d) FORCE I OUTPUT FROM ROLL FORCE 27 (e) NETWORK I l GOMPENSATED 27o 7 GAUGE (1) MEASUREMENT FIG.4

4 Sheets-Sheet 5 TIME TIME

TIME

TIME

TIME

TIME

TIME

TIME

16, 1966 w. G WRIGHT 3,266,279

AUTOMATIC GAUGE CONTROL SYSTEM FOR ROLLING MILLS Filed Oct. 30, 1963 4 sheets sheet 4 p TRANSPORT DELAY I m K" 39 (a) f TIME TIME Y MEASURED 4| GAUGE I TIME ROLL 4 (a) FORCE mas ' |v OUTPUT'FROM 'v ROLLFORCE NETWORK (e) O TIME l UP' (f) scREw TIME SPEED 7 DOWN 44 I OUTPUTFROM (m TIME TACHOMETER O y NETWORK FIGS I United States Patent 3,266,279 AUTOMATIC GAUGE CONTROL SYSTEM FOR ROLLING MILLS William G. Wright, Peterborough, Ontario, Canada, assignor to Canadian General Electric Company, Limited, Toronto, Ontario, Canada, a corporation of Canada Filed Oct. 30, 1963, Ser. No; 320,110 7 Claims. (Cl. 728) This invention relates to apparatus for rolling materials such as steel into strip form, and in particular to means for controlling the apparatus so as to obtain finished strip of uniform thickness.

Metals such as steel are usually rolled into strip or sheet form in two stages, viz., an initial stage where the metal is rolled while hot to a thickness somewhat in excess of the final thickness specified, and a second stage where the unfinished strip is rolled while cold to its final dimension. Since the unfinished strip may vary in thickness along its length, the controls for the cold mill should included some means to prevent the mill from simply duplicating these variations in thickness of the strip. In other words, the cold mill should be compensated in such a way that during the time that the rolls of the mill reduce the thickness of the strip they also smooth out these variations and produce a finished strip of uniform thickness, that is, uniform within specified tolerances.

It is old in the art to measure the thickness of the finished strip as it leaves the rolls by means of a gauge such as a flying micrometer, and to employ an operator to observe these measurements and continually adjust the mill accordingly so as to produce a finished strip as uniform in thickness as possible. The adjustments made by the operator to the mill will be one or both of the following: the separation between the rolls, and the tension applied to the strip during rolling thereof. In this method of control the skill of the operator determines to a large extent the uniformity in thickness of the finished strip. However, with the introduction of equipment for using the micrometer readings to control the mill automatically, the accuracy of the mill is no longer limited by the skill of the operator.

With the increase of speeds in modern mills, another significant source of error in the control of thickness of the strip arises from the location of the flying micrometer with respect to the rolls of the mill. Unless the measurements are taken on the strip at the point where it leaves the rolls (often referred to as the roll bite), a delay in time is introduced into the control system. Because it isphysically impossible to place the micrometer in this position, the strip must move from between the rolls to I the micrometer before its thickness is measured. This time delay varies inversely with the speed of the mill and it may become excessive at low mill speeds. For example, the time delay will be one second when the micrometer is located three feet from the roll bite and the strip is travelling at a rate of 180 feet per minute. This time delay is well known by the expression transport delay and it will be referred to hereinafter as such. Transport delay introduces errors into the controls for the mill, causes instability of the system, and renders control of gauge very difficult, particularly during acceleration of the mill,

A gauge control system developed by the British Iron and Steel Research Association (BISRA system) uses the mill itself as a means for determining the thickness of the strip. In one version of the BISRA type of system, two electric signals are added together to give a combined signal representing the loaded roll separation or the approximate thickness of the strip leaving the rolls. The first of the two signals represents the separation ice of the unloaded rolls, and it may be obtained from a device such as a potentiometer actuated by the screws used to position the rolls. The second signal represents the forces produced in the frame of the mill by the loading on the rolls from the strip passing between the rolls, and this signal may be derived from one or more load cells attached to the mill at loaded regions thereof, the theory being that the frame of the mill obeys Hookes Law. This combined signal is then compared with an electric signal representing the thickness of the strip desired, i.e., a reference obtained from a constant voltage source by means of a potentiometer. A comparison of the two signals leads to an error signal which represents the extent to which the aforementioned approximated thickness of the strip deviates from the thickness desired. The error signal is applied to the gauge control system, and it causes the controls for the mill to correct the error by either altering the position of the rolls, by altering the tension on the strip, or by altering the position of the rolls and the tension on the strip simultaneously. It is to be noted that the combined signal is taken in the region where the rolls contact the strip, and as a result the transport delay has been eliminated. However, even with the BISRA system it is still necessary to make use of a thickness measuring instrument such as a micrometer. In this case, the role of the micrometer is to correct for slow drifts in accuracy which occur due to such things as roll wear and thermal expansion.

The object of my invention is to provide a gauge control system which also effectively eliminates the transport delay, but does not depend on the precise measurements of roll force and roll position necessary for the BISRA system.

My invention is practiced by making use of certain anticipatory signals available at the mill, and it will be described now with reference to the attached drawings in which:

ISIGURE 1 is a schematic diagram of a metal rolling m1 FIGURE 2 is a diagram in block form of a control system constructed in accordance with my invention; and

FIGURES 3, 4 and 5 illustrates graphically signal forms used to explain the nature of the invention.

In FIGURE 1 there is shown a strip of metal 2 entering between the main rolls 3 and 4 of a four-high rolling mill at a thickness of h inches and leaving these rolls at a reduced thickness of k inches. The thickness of the strip after reduction thereof is measured by means of an instrument 5, such as an X-ray thickness gauge, located at a distance L feet from the bite of the rolls, the bite of the rolls lying on the dotted line 6 passing through the axes of the rolls. Rolls 3 and 4 are supported for rotation in the mill frame between a pair of backing rolls 7 and 8 respectively, and the distance between rolls 3 and 4 can be adjusted by rotating two mills screws, of which one is shown at 9 threaded through a nut 10 fixed to the mill frame. Axial movement of a screw is transmitted to backing roll 7 through a load cell 11, a column 12 and a bearing 13. Screw 9 is rotated by a gear 14 secured to its upper end, and the gear is driven by a motor 15 through a train of gears 16, 17 and 18 respectively. A tachometer 19 coupled to the motor senses its speed and direction of rotation. FIGURE 1 is a schematic arrangement only of those components of a mill helpful in demonstrating the nature of my invention. In a practical form of this arrangement, there are two assemblies of components 9 to 18, one assembly at each end of therolls. The two screw down drives are coupled together mechanically through a clutch such that under normal operation of the mill the two motors 15 operate in unison, With the speed of the drive measured by a single tachometer. However, release of the clutch allows the motor to be operated independently to position rolls 3 and 4 in parallel should this be necessary. The thickness measuring instrument 5, tachometer 19 and load cells 11 are all known types of instruments which give an electrical output representing the particular quantity being measured. Since the load cells are mounted between the top backing roll and the screws, they are subject to the forces applied to the rolls for reducing the thickness of the strip, i.e., the roll separating forces. The electrical signals from the two load cells are summed and used in a roll force network, and the electrical signals from the tachometer are used in a tachometer network, both of which will be described later. Since the operation of the system will be the same with one or two load cells, one cell only will be considered in the description to follow in the interest of simplicity.

As the first step in explaining the nature of the inven- 'tion, let it be assumed that the mill screws are stationary. Therefore, so long as strip 2 entering between rolls 3 and 4 is uniform in thickness I1 and hardness, the strip leaving the rolls will also be uniform in thickness h and the roll separating force will remain at a constant value.- Since the output from load cell 11' is directly proportional to the roll separating forces, this signal will have a constant value, i.e., the DC. voltage represented by line 22 in FIGURE 3(a). If the thickness h of the incoming strip increases at the point 23 or if the hardness of the strip increases at this point, the roll separating forces will increase abruptly and cause an immediate increase in signal output from'the load cell as indicated at 24. The output of the load cell rises to a new level 25 representing the new conditions, where it will remain until the roll separation forces are disturbed again; Signal 24 from the load cell represents a rate of change of roll separating forces and indicates a change of gauge h The output from load cell 11 is amplified by amplifier 21 and then applied to a resistor capacitor circuit, i.e., the resistor R capacitor C circuit shown as a part of the roll force network in FIGURE 2. So longas the output from the load cell is constant in magnitude, the output from the R C circuit will be zero. However, any change in signal, such as the abruptv change 24, will alter, the charge on capacitor C and consequently, there will be an output from the R C circuit having the usual exponential wave form characteristic of RC circuits.

FIGURE 3 shows that when a change of roll force takes place, the resistor capacitor network will have an output which will decay as illustrated by curve 27. As is-well known, the time constant of the R circuit will equal the resistance of R in megohms multiplied by the capacitance of C in microfarads. To make effective use of the output from the resistor capacitor network in a gauge control system to compensate for transport delay,

the time constant of R C must be suitably related to the transport delay of the mill; the relationship requires that R C be equal to one-half the transport delay. It can be shown mathematically that this is actually the case, that R 0 can be made to equal one half the transport delay of the mill by proper selection of resistor and capacitor components. Since the transport delay varies inversely with mill speed, it follows that R should also vary inversely with mill speed. R can be made to vary in this way by using a variable resistor and coupling it mechanically to a servo 28 which follows mill speed. Therefore, R is adjusted continually so that R C =V2 transport delay. Transport delay=L/ V seconds, where L is the distance in feet of the thickness measuring instrument from the bite of the rolls and V is the velocity in feet per second of the strip through the mill.

The rate of change of roll force signal provides a means for discovering that a change of gauge has taken place before the thickness of the strip is actually measured. This is illustrated graphically in FIGURE 4. Still assuming that screws 9 are stationary, the mill is reducing a strip of thickness h to a thickness of k as denoted at 29 i and 30 respectively in FIGURES 4(a) and (b). At point 23 the thickness of the incoming strip increases to 33m sulting in a simultaneous increase in thickness of the outgoing strip to 34. However, the increased thickness 34 is not measured until some time later at 32 because the measuring instrument is located L feet downstream from the bite of the rolls. Hence it is impossible for the measuring instrument to initiate corrective measures until some time after the change of gauge has taken place, i.e., the transport delay later as shown in FIGURE 4(c). It will be noted with reference to FIGURE 4(d) that at 23 where h increases, the roll force increases as well from 31 to 35; therefore, the signal from the load cell changes as already described with reference to FIGURE 3, cansing the roll force network to give the signal illustrated graphically in FIGURES 3(b) and 4(e). Since signal 27 represents a rate of change of roll force rather than a measure of the correction required to bring the mill back on gauge, it can only be used to initiate screwdown of the rolls, after which other means must be introduced to avoid overcorrection'. In FIGURE 4(f) numeral 36 denotes a signal representing the thickness 30 measured by gauge 5 before the roll force increased at 23. If the output from the roll force network is added to signal 36, the sum will appear as 27a, i.e., as signal 27 superposed on signal 36. At 32, the increase in thickness of the strip is actually measured, resulting in an increase of signal output from the thickness measuring instrument. This increase in gauge signal raises the level of curve 27a to 37. FIGURE 4(f) shows a compensated gauge measurement equal to the measured gauge plus the output from the roll force network. A greater degree of control is possible when the output from the roll force network is combined with the output from the thickness measuring instrument, but this combination is insufiicient for full control of gauge because the screwdown mechanism must be considered too.

It has been shown that signals derived from a rate of change of roll forces can be used to detect variations in thickness of the strip when these variations occur. The next step to consider is the effects of screw movement on the control of gauge. In order to simplify the analysis to follow, let it be assumed that there are no variations in thickness or hardness of the incoming strip to alter the roll separating forces. Instead the strip is reduced in thickness by running the rolls down, and as the separation between the rolls decreases, the roll forces will increase because more metal must now be displaced by the rolls. Therefore, the signal appearing at the roll force network will be due to an increase of roll separating force as was the signal obtained when the thickness of the incoming strip increased. In other Words, the output from theroll force network due to an increase in thickness of the incoming strip is in the same direction as the output from this network due to the running down of the screws, even though in the first instance the thickness of the outgoing strip increased while in the second instance the thickness of the outgoing strip decreased. Therefore, as soon as the screws move it is insuificient to use roll force alone as an anticipatory signal, a signal suitably related to the rate of change of screw position must also be injected into the system for controlling roll separation.

Actually, the rate of change of screw position is its speed which can be measured by tachometer 19. A tachometer having a D0. output will sense direction of rotation of the screw as well as measure speed of rotation thereof. As with the signal from the load cell, the signal from the tachometer must be suitably related to the transport delay of the mill before it is combined with the output from the roll force network for control of the mill. To this end it can be shown mathematically that the tachometer signal should be conditioned by a resistor capacitor network wherein the time constant is again equal to one half the transport delay. It will be seen from FIGURE 2 that the signal from tachometer 19 is applied to the network R C and the output from this network is fed to amplifier 20 where it is amplified. Since R C is the same type of resistor capacitor network as R C it too will have an output signal only when the signal from the tachometer is changing in value, i.e., when the screws are either accelerating or decelerating. In practice, roll adjustments are very small and very frequently in opposite directions, hence the screws are either accelerating or decelerating when in motion. Tachometer 19, resistor R capacitor C and amplifier 20 will be referred to hereinafter as the tachometer network. The resistor capacitor network R C serves the same purpose in the tachometer network that network R serves in the roll force network, and moreover, resistor R is coupled mechanically to servo 28 in the same way as resistor R Therefore, R is adjusted continually in accordance with mill speed so that R C /2 transport delay.

FIGURE (a) illustrates a strip of uniform thickness I1 passing between the rolls when at point 38, the screws are turned down to reduce the thickness k of the strip leaving the rolls as illustrated in FIGURE 5(b), the thickness of the outgoing strip being measured at a time later equal to the transport delay as illustrated in FIG- URE 5(0). As soon as the screws are turned down, the roll force increases as illustrated in FIGURE 5 (d). Since the screwdown operation is not instantaneous, reduction in thickness is gradual with respect to time as indicated at 40 and 41, and as a result the roll force signal from the load cell will tend to rise along a slope 42 related to the rate of reduction in thickness. After the roll force signal has been modified by the RC circuit of the roll force network it'will appear as curve 43 in FIGURE 5(a). In FIGURE 5(f), curve 44 represents a signal from the tachometer while the screws are accelerating and curve 45 the signal while the screws are decelerating. The output from the tachometer network will appear as curve 46 in FIG. 5 (g). Curves 43 and 46 may be combined to give a signal which shows the way in which the thickness of the strip is changing. In this particular example it will be noted that curves 43 and 46 are in opposite sense and that curve 46 is the larger of the two,

thus indicating that the thickness of the strip is getting less.

The signal obtained from the roll force network indicates whether roll force is increasing or decreasing, and represents the rate at which the force is changing. The signal obtained from the tachometer network indicates the direction of screw movement and represents the rate at which the screws are adjusting the position of the rolls. By summing these two signals and comparing the sum thereof with thesignal obtained from the thickness measuring instrument, a combined signal is obtained having the correct sense under all conditions for the application to the control of gauge; and moreover this combined signal has virtually eliminated the transport delay.

Referring now to FIGURE 2, the signal from the tachometer network is fed through potentiometer R to summing amplifier 47, where it and the signal from the roll force network are summed and amplified. Potentiometer R is coupled mechanically to servo 28 so as to continually adjust the gain of the signal from the tachometer network, rendering the gain directly proportional to the speed of the mill. The signal output from amplifier 47 is filtered at 48 to remove the effects of eccentricity of the backing rolls, and it is then combined with the signal from thickness measuring instrument 5 in summing amplifier 49. The output from amplifier 49 is composed of three separate signals, the first two of which have been modified in such a way that the composite signal is suitable for application to the controls for the mill. In a practical system, it is likely that the output signal from the micrometer will represent deviations from a set value rather than actual thickness of the strip. Thus, the output from amplifier 49 will be a signal proportional to thickness deviations. Therefore, the expression an electrical signal representative of the thickness being measured appearing in the claims can be a signal representing the thickness of the strip as actually measured or a signal representing a measured deviation from a thickness value set on the measuring instrument. The additional signal from the load cells and tachometer may be regarded as stabilizing signals which compensate for the effect of the transport delay.

In summary, my system differs in some fundamental respects from prior art systems in that I overcome the transport delay without accurately determining the separation of the rolls or the roll forces. I control gauge through the medium of anticipatory signals obtained at the mill and modified to render them suitable for control purposes.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In apparatus for performing work upon a moving strip by a pair of rolls between which said strip is advanced; gauging means positioned along the path of said strip at some distance from the bite of said rolls for measuring the thickness of the strip after it leaves the rolls and for producing an electrical signal representative of the thickness being measured; said gauging means measuring the thickness of the strip at a time interval after reduction thereof and thereby introducing a transport delay in the control system; means for generating a first electrical signal representative of the rate of change of roll separating force; first circuit means having a time constant approximately equal to one half said transport delay for converting said first signal to a decaying signal having a generally exponential wave form; means for generating a second electrical signal representative of the rate of change of separation of said rolls; second circuit means having a time constant approximately equal to one half said transport delay for converting said second signal to a decaying signal having a generally exponential wave form; and means for summing said exponential signals and comparing the sum thereof with the electrical signal from said gauging means, the signal output from said last mentioned means substantially eliminating the effects of said transport delay and therefore a signal useful in said control system for controlling gauge automatically.

2. In apparatus for performing work upon a moving strip by a pair of rolls between which said strip is advanced; gauging means positioned along the path of said strip at some distance from the bite of said rolls for measuring the thickness of the strip after it leaves the rolls and for producing an electrical signal representative of the thickness being measured; said gauging means measuring the thickness of the strip at a time interval after reduction thereof and thereby introducing a transport delay in the control system; means for generating a first electrical signal representative of the rate of change of roll separating force; a first resistor capacitor network having a time constant approximately equal to one half said transport delay for modifying said first signal; means for generating a second electrical signal representative of the rate of change of separation of said rolls; a second resistor capacitor network having a time constant approximately equal to one half said transport delay for modifying said second signal; and means for summing said modified signals and comparing the sum thereof with the electrical signal from said gauging means; the signal output from said last mentioned means substantially eliminating the effects of said transport delay and therefore a signal useful in said control system for controlling gauge automatically.

3. The apparatus defined by claim 2 wherein one or both of the time constants of said first and said second circuit means are varied with changes of roll speed to compensate for variations in transport delay with such changes of roll speed.

4. The apparatus defined 'by claim 2 wherein at least by roll eccentricity.

6. The apparatus defined by claim 2 wherein amplifying means is provided for amplifying said signals, gain control means is provided for rendering the amplitude of said second modified sign-a1 proportional to the speed of the rolls, and filter means is provided for removing extraneous signals caused by roll eccentricity.

7. In apparatus for performing work upon a moving strip by a pair of rolls between which said strip is advanced; gauging means positioned along the path of said strip at some distance from the bite of said rolls for measuring the thickness of the strip after it leaves the rolls and for producing an electrical signal representative of the thickness being measured; said gauging means measuring the thickness of the strip at a time interval after reduction thereof and thereby introducing a transport delay in the control system; means for generating a first electrical signal representative of the rate of change of roll separating force, means for amplifying said first signal; a first resistor capacitor network having a time constant approximately equal to one half said transport delay for modifying said amplified first signal; means for gen- 8 crating a second electrical signal representative of the rate of change of separation of said rolls; a second resistor capacitor network having a time constant approximately equal to one half said transport delay for modifying said second modified signal; gain control means following the speed of the wells for adjusting the gain of said amplifier means for rendering the amplitude of the signal therefrom proportional to the speed of the rolls; means for summing the signals from said first resistor capacitor network and from said gain control means; filter means for filtering said combined signal for removing extraneous signals caused by roll eccentricity; means for summing the signal from said filter means and the signal from said gauging means; the signal output from said last mentioned means substantially eliminating the effects of said transport delay and therefore a signal useful in said control system for controlling gauge automatically; and means following the speed of said rolls for varying the resistances of said resistor capacitor networks to maintain the time constants thereof equal to approximately one half said transport delay which also varies with roll speed.

References Cited by the Examiner CHARLES W. LANHAM, Primary Examiner.

R. D. GREFE, Assistant Examiner. 

1. IN APPARATUS FOR PERFORMING WORK UPON A MOVING STRIP BY A PAIR OF ROLLS BETWEEN WHICH SAID STRIP IS ADVANCED; GAUGING MEANS POSITIONED ALONG THE PATH OF SAID STRIP AT SOME DISTANCE FROM THE BITE OF SAID ROLLS FOR MEASURING THE THICKNESS OF THE STRIP AFTER IT LEAVES THE ROLLS AND FOR PRODUCING AN ELECTRICAL SIGNAL RESPRESENTATIVE OF THE THICKNESS BEING MEASURED; SAID GAUGING MEANS MEASURING THE THICKNESS OF THE STRIP AT A TIME INTERVAL AFTER REDUCTION THEREOF AND THEREBY INTRODUCING A TRANSPORT DELAY IN THE CONTROL SYSTEM; MEANS FOR GENERATING A FIRST ELECTRICAL SIGNAL REPRESENTATIVE OF THE RATE OF CHANGE OF ROLL SEPARATING FORCE; FIRST CIRCUIT MEANS HAVING A TIME CONSTANT APPROXIMATELY EQUAL TO ONE HALF SAID TRANSPORT DELAY FOR CONVERTING SAID FIRST SIGNAL TO A DECAYING SIGNAL HAVING A GENERALLY EXPONENTIAL WAVE FORM; MEANS FOR GENERATING A SECOND ELECTRICAL SIGNAL REPRESENTATIVE OF THE RATE OF CHANGE OF SEPARATION OF SAID ROLLS; SECOND CIRCUIT MEANS HAVING A TIME CONSTANT APPROXIMATELY EQUAL TO ONE HALF SAID TRANSPORT DELAY FOR CONVERTING SAID SECOND SIGNAL TO A DECAYING SIGNAL HAVING A GENERALLY EXPONENTIAL WAVE FORM; AND MEANS FOR SUMMING SAID EXPONENTIAL SIGNALS AND COMPARING THE SUM THEREOF WITH THE ELECTRICAL SIGNAL FROM SAID GAUGING MEANS, THE SIGNAL OUTPUT FROM SAID LAST MENTIONED MEANS SUBSTANTIALLY ELIMINATING THE EFFECTS OF SAID TRANSPORT DELAY AND THEREFORE A SIGNAL USEFUL IN SAID CONTROL SYSTEM FOR CONTROLLING GAUGE AUTOMATICALLY. 